A refrigeration oil composition obtained by dissolving a refrigerant in a refrigeration oil is used as a working fluid in the refrigeration cycle in automotive air conditioners, domestic air conditioners, air conditioning within buildings, cold storehouses, refrigerators, and the like, and hydrofluorocarbon-based refrigerants which contain no chlorine and are constituted of hydrogen, carbon, and fluorine are in use from the standpoint of environmental conservation. Polyol ester oils are used accordingly as refrigeration oils for dissolving the hydrofluorocarbon-based refrigerants therein (see, for example, patent document 1).
However, even refrigeration oil compositions containing the same hydrofluorocarbon-based refrigerant usually differ in requirements concerning the temperature at which the composition separates into the refrigeration oil and the refrigerant (two-layer separation temperature), kinematic viscosity, etc. depending on applications or purposes. Although it is generally necessary to lower the kinematic viscosity of a polyol ester oil for increasing the solubility of a hydrofluorocarbon-based refrigerant therein, the reduced kinematic viscosity results in a decrease in the lubricity of the refrigeration oil composition. Meanwhile, in cases when the lubricity is improved by heightening the kinematic viscosity of the polyol ester oil, the hydrofluorocarbon-based refrigerant comes to separate out.
There are hence cases where a polyol ester oil which is excellent in terms of the solubility of hydrofluorocarbon-based refrigerants therein and a polyol ester oil having excellent lubricity are mixed with each other and used so as to result in solubility and kinematic viscosity that are suitable for the application or purpose (see, for example, patent documents 2 and 3). However, in case where the two polyol ester oils differ considerably in solubility with refrigerants, the polyol ester oil which is less soluble in the refrigerant gradually separates out, depending on the refrigerant used, during the period when the refrigeration oil composition circulates in the refrigeration cycle.
Meanwhile, since hydrofluorocarbon-based refrigerants have an exceedingly high global warming potential, hydrocarbon-based refrigerants, which are considerably low in global warming potential although flammable, are employed in some applications. There is a possibility that such hydrocarbon-based refrigerants might be increasingly used in various applications in future so long as apparatus modification measures are taken to cope with the problem concerning flammability. As refrigeration oils for dissolving hydrocarbon-based refrigerants therein, use has been mainly made of naphthenic or paraffinic mineral oils, alkylbenzene oils, ether oils, fluorinated oils, and the like. However, polyol ester-based lubricating oils having come to be used for the purpose of further improving lubricity. For example, an ester oil which is an ester of a neopentyl polyol including neopentyl glycol or pentaerythritol with a branched monovalent fatty acid having 7 to 9 carbon atoms (see patent document 4) and a polyol ester oil having an alkyl group having 11 to 19 carbon atoms (see patent document 5) are in use.
However, as in the case of hydrofluorocarbon-based refrigerants, such refrigeration oils usually differ in requirements concerning two-layer separation temperature, kinematic viscosity, etc. depending on the applications or purposes of the apparatus to be used. In addition, since hydrocarbon-based refrigerants have an exceedingly low density which is about one-half the density of hydrofluorocarbon-based refrigerants, the volume concentration of a hydrocarbon-based refrigerant dissolved in a refrigeration oil even in the same weight concentration as before is twice and the viscosity, which is influenced by volume concentration, is considerably lower than the conventional refrigerant-solution viscosity. This decrease in refrigerant-solution viscosity is a factor which causes a decrease in lubricity. In the case of using a refrigerant and a refrigeration oil which have high solubility with each other, the refrigeration oil to be used is required to have high kinematic viscosity from the standpoint of heightening the refrigerant-solution viscosity. However, since the refrigerant and the refrigeration oil considerably differ in density and because the refrigeration oil having too high a viscosity has impaired flowability, the refrigerant and the refrigeration oil are less apt to mingle with each other in an actual machine, resulting in a possibility that the refrigeration oil might be supplied in a reduced amount to the surfaces to be lubricated and the lubricity be impaired rather than improved.
However, the refrigeration oil described in patent document 4 has a low viscosity (about 10 to 32 mm2/s at 40° C. according to Examples) and may raise difficulties when used in large refrigerated facilities. Furthermore, the refrigeration oil described in patent document 5 is intended to be used with R290, and is usable only in limited applications.
Meanwhile, in production sites for producing refrigeration oils, it is necessary that many kinds of polyol ester oils produced from a large number of raw materials are ready for use so as to be capable of coping with differences in applications or purposes and in the kinds of hydrofluorocarbon-based refrigerants, and this has resulted in complexation of procurement of raw materials, production management, and product control. The polyol ester oils described in patent documents 2 and 3 also are each intended to be suited for specific refrigerants and specific apparatus, and when used in combination with other refrigerants or in other apparatus, show too high or too low solubility with the refrigerants or have too high or too low viscosity. In particular, it is impossible to obtain suitable solubility with less soluble refrigerants, such as difluoromethane (R-32) refrigerant, while ensuring necessary viscosity.
The same applies to hydrocarbon-based refrigerants. Although it is necessary that many kinds of refrigeration oils produced from a large number of raw materials are ready, the current prior-art techniques including patent documents 4 and 5 are ineffective in overcoming such problems.